The present invention relates generally to optical resonator devices and, more particularly, to a radial Bragg ring resonator structure with a high quality factor, Q.
Optical resonators comprise the central components of light source devices such as, for example, high-efficiency light emitting diodes, lasers, switches, and filters. In the field of computing, multi-core microprocessor architectures have been developed in order to mitigate increased power dissipation in high-performance computer chips. However, the bandwidth limitations for global electrical interconnections between various cores are rapidly becoming the major factor in restricting further scaling of total chip performance. One approach to resolving this interconnect bottleneck is to transmit and route signals in the optical domain, since optical signals can provide both immense aggregate bandwidth and large savings in on-chip dissipated power. As such, optical resonators are desirably integrated with integrated circuit (IC) device substrates. In fact, the field of integrated optics has expanded tremendously in recent years, and integrated optical device solutions are now being proposed for applications in a variety of fields including, for example, telecommunications, data communications, high performance computing, biological and chemical sensing, and radio frequency (RF) networks.
Specific solutions for optical resonators that are may be integrated on planar substrates (e.g., silicon) include structures such as, for example, linear resonators with distributed Bragg reflector mirrors, individual ring or disc resonators, photonic crystals and radial Bragg ring resonators. In particular, radial Bragg ring resonators, which are also known as circular grating resonators (CGRs) or “fingerprint” structures, have more recently been considered for applications in integrated optics such as lasing and all-optical switching. Radial Bragg ring resonators have a very small footprint of a few micrometers, which essentially corresponds to the smallest optical resonators possible. Thus, even at relatively low refractive index contrasts, radial Bragg ring resonators offer full two-dimensional light confinement, making them a very attractive candidate for future integrated photonic devices since they may be fabricated of any transparent (low absorption) material.
Two of the primary parameters of interest for optical resonators having a resonance at a certain wavelength are the quality factor, Q, and the effective mode volume, Veff. With respect to both linear resonators having distributed Bragg reflector mirrors and disc resonators, a primary disadvantage thereof is the large mode volume of those resonators, wherein Veff is on the order of hundreds of times the operating wavelength of light cubed (λ3). This in turn leads to low optical confinement, prevents dense areal integration and requires relatively high power to drive optically active material inside the optical resonator. Furthermore, integrated devices which harness cavity quantum electrodynamic effects such as the Smith-Purcell effect (e.g., single-photon sources) or the photon blockade regime (e.g., single-photon switches) are not possible as they require a large ratio of Q/Veff.
With respect to photonic crystals, the primary drawback of this type of resonator is that it requires a large refractive index contrast between the material of the photonic crystal (such as GaAs or Si) and the surrounding material (such as air) in order to achieve a complete two-dimensional bandgap. This limits their use in terms of wavelengths (infrared), fabrication (suspended membranes) and materials (i.e., semiconductor materials which are not back end of line (BEOL)-compatible in their crystalline form).
In addition, the aforementioned linear resonators having distributed Bragg reflector mirrors, disc resonators, and photonic crystals all share the disadvantage that resonator modes with a dipole-like mode profile are not known. This, however, is required for optimum coupling to (for example) molecules, nanoparticles and quantum dots, which are potentially located in the resonator serving as absorbing, emitting or non-linear material. Finally, a primary drawback of conventionally designed radial Bragg ring resonator is the low quality factor Q (e.g., <10000) for devices having small mode volumes (e.g., on the order of a few λ3), which is caused by large vertical or out-of-plane losses.